Generally, complex titanium structures can be made by machining a block of titanium to provide the desired structure, which is wasteful, time-consuming and expensive. Alternatively, such structures can be made by forging. The present invention provides a simpler and cheaper method for making such structures.
Combined diffusion bonding and superplastic forming is an established technique for making composite articles from materials which exhibit superplastic properties at elevated temperatures. These materials are primarily titanium, aluminium and alloys of both these metals. In established DB/SPF processes, e.g. as described in U.S. Pat. No. 5,143,276, it is known to apply stop-off material to selected areas of two or more sheets of superplastic material; several sheets, including the sheets to which stop-off material has been applied, are then assembled into a pack with the stop-off material lying between adjacent superplastic sheets. The assembled pack is then heated and compressed until the sheets are diffusion bonded together: however, the sheets are not bonded in the selected areas covered by stop-off material since the stop-off material prevents diffusion bonding in those areas. The superplastic forming step is then conducted by heating the bonded pack, usually in a mould to a temperature at which the components exhibit superplastic properties. An inert gas is then injected in a controlled manner into the unbonded areas of the pack under high pressure so as to "inflate" the sheets gradually into a three dimensional structure having an outer shape corresponding to the shape of the mould. The configuration of the final composite structure is dependent upon, among other things, the number of sheets in the pack, the location of the stop-off material and the shape of the mould.
It is known, for example from GB-1495655, to form a composite panel from a pack comprising a pair of opposed face sheets and a core sheet sandwiched between, and bonded to, the face sheets; in the superplastic forming process, the face sheets are forced apart and because the internal core sheet is attached to both of the face sheets, the core sheet adopts a zigzag shape that, in effect, constitutes struts extending from one face sheet to the other.
U.S. Pat. No. 4,304,821 and U.S. Pat. No. 5,143,276, each describes the making of a panel from four sheets of superplastic material from a pack comprising a pair of opposed face sheets and two core sheets sandwiched between the face sheets; the two core sheets are bonded to each other by linear welds. The face sheets are superplastically formed by injecting gas into the area between each face sheet and the adjacent core sheet to expand the face sheets into the shape of a mould; gas is then injected between the two core sheets. Because the core sheets are joined by the linear welds, the core sheets expand to form cells extending between the face sheets; the side walls of the cells are formed by U-shaped doubled-back sections of the two core sheets.
GB-4129340, GB-2030480, U.S. Pat. No. 4,534,503, U.S. Pat. No. 4,607,783, U.S. Pat. No. 4,351,470 and EP-0502620 all disclose methods of forming hollow panels using DB/SPF techniques but none of them disclose the manufacture of a structure having cantilever ribs.